1. Field of Invention
The present invention relates generally to controlled braking mechanisms, and more particularly, to a variable torque braking apparatus and method for use in conjunction with escalators, moving walkways, and the like.
2. Related Art
Current braking systems for escalators and moving walkways (also known as, e.g., power walks or travelators) include, for example, open control loop braking systems with guided compression springs. Alternatively or in addition, closed control loop braking systems with ceramic magnetic brakes have been utilized. Each of these braking systems, however, present a variety of control issues.
For example, open control loop systems with guided compression springs are bi-stable braking devices. That is, the brake is released during starting and running and applied with a constant spring force when the escalator or power walk is stopped. Due to this bi-stable functionality, the stopping distance and stopping rates can vary widely depending on escalator loading. For example, a lightly loaded escalator will stop in less time and over a shorter distance than a fully loaded escalator.
To reduce the stopping distance differences between no load and full load conditions on the escalator, a large flywheel is often added to the motor to offset the effects of escalator loading. Even though the large flywheel can provide the inertia to prevent the escalator from stopping too quickly when the escalator is lightly loaded, this same inertia is counterproductive when the escalator is fully loaded. The large flywheel requires that the brakes provide enough torque to stop the load on the escalator as well as to stop the large flywheel. Thus, the presence of the flywheel requires the brakes to be oversized for all applications.
Moreover, notwithstanding the addition of a large flywheel, the difference in stopping distance and rate can still vary between no load and full load conditions. In many applications, this difference in stopping distances can still pose a problem if the customer requires that the stop distance range be narrower than the open loop brake system can provide. Additionally, in the United States and Canada, the ASME A17.1 Escalator Safety Code limits maximum deceleration rates as well as maximum stopping distances.
Another problem that can arise with open loop spring applied braking systems is that the available brake torque may diminish over time due to brake wear or environmental conditions. As a result, the stopping rates and distances can become non-code compliant and/or may not meet customer specifications if the brake is not readjusted or replaced regularly.
In an effort to provide consistent stopping, some manufacturers of escalators and moving walkways have added inverters (e.g., AC drives) to provide dynamic motor braking. The addition of inverters, however, can also have disadvantages. Inverters can be costly and can require extra room in the escalator truss for mounting. This can be prohibitive on some types of escalators where there is no extra room in the truss. Also, at least one of a dynamic resistor or a regeneration unit is required to dump the generated braking energy. Both items also add cost and require space for mounting.
Moreover, it is generally not practical to add closed loop control to guided compression spring braking systems because the brake coils on these types of units are bi-stable devices. Bi-stable devices are designed to be either actuated or not actuated. Consequently, controlling the brake linearly is not possible through closed loop control.
Although ceramic magnetic brakes utilizing closed control loop braking may solve many of the problems inherent in guided compression spring open control loop braking systems, they nevertheless present other issues. For example, while stopping distances and rates achieved using closed loop controlled magnetic braking systems may be much more consistent than with open loop spring applied braking systems, the magnetic brake can tend to be sluggish. Consequently, the braking system can be relatively slow to hone in on a specific braking torque required for a given escalator load. The result is that the stopping rate may either under shoot or over shoot a set point at the beginning of the stop sequence, thereby producing a “wavy” stop until the control is able to hone in on the correct torque.
Furthermore, since the magnetic null characteristics of magnetic brakes can vary from brake to brake, it is necessary to tune the closed loop brake controller for each brake before putting the brake into service. If the brake controller is not tuned, then it is possible for the brake to drag slightly over time and/or provide a stop that is not optimum. Also, the use of a magnetic brake does not allow the addition of a second brake on the same motor. As a result, magnetic brakes cannot be used for certain applications, particularly in Europe where the European escalator code (EN code) requires the use of compression guided springs as well as a second brake whenever the rise of the escalator exceeds a certain height.